Low level costant current source



Oct. 27, 1970 D. E. P-ERLMAN 3,536,986

LOW LEVEL CONSTANT CURRENT SOURCE Filed Dec. 18, 1968 DAVID E. PERLMAN ATTORNEYS United States Patent 3,536,986 LOW LEVEL COSTANT CURRENT SOURCE David Eric Perlman, Rochester, N. assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Filed Dec. 18, 1968, Ser. No. 784,758 Int. Cl. GF 1/56; H02] 1/04 US. Cl. 323-4 4 Claims ABSTRACT OF THE DISCLOSURE A low level current source has resistances which are small enough so that the circuitry can be manufactured as a monolithic integrated circuit.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to constant current sources and more particularly to constant current sources which are adaptable for supplying currents on the order of less than one microampere.

Description of the prior art SUMMARY OF THE INVENTION It is therefore an object of the invention to substantially obviate the above disadvantages and to provide a low level constant current source which exhibits good regulation and which can be economically fabricated as an integrated circuit by virtue of its having no high value resistances.

The above and other objects of the invention are attained by providing a stage of amplification to increase the flow of current through the feedback resistance so that the amount of current flowing through the feedback resistance is enough so that the feedback resistance can be of a much lower value in order to provide the same voltage drop with a low resistance value that previously was available in the prior art only with a high resistance value.

BRIEF DESCRIPTION OF THE DRAWING The invention will be better understood when reference is made to the following specification taken with the accompanying drawing wherein:

The sole figure shows a schematic diagram of one embodiment of a constant current source according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing there is shown a unidirectional current device such as NPN transistor Q The control or base terminal 1 of transistor Q; is electrically connected to the collector terminal 2 of transistor Q so that transistor Q operates substantially as a diode. The emitter terminal of transistor Q, is electrically connected to common terminal 4 which is grounded. The collector of transistor Q, is connected through resistor 3 to the positive terminal of a voltage source S. The collector terminal of transistor Q, is connected to the control terminal of a unidirectional current control device such as the base of "ice NPN transistor Q The circuitry thus far described keeps a substantially constant voltage on the base of transistor Q during variations in voltage of the source S since the voltage drop across transistor Q when the latter is connected as a diode, stays substantially constant over a wide range of current values.

The collector terminal 6 of transistor Q is coupled to the control termnial of a unidirectional current control device such as the base terminal 7 of PNP transistor Q The emitter terminal of transistor Q is coupled to the constant current output terminal I while the collector terminal 9 of transistor Q is coupled to the control terminal of a unidirectional current control device such as the base terminal 10 of NPN transistor Q The emitter terminal 11 of transistor Q and the emitter terminal 12 of transistor Q are coupled together to resistor 5. The collector terminal 13 of transistor Q; is also coupled to the positive terminal of the voltage source.

The current which flows at output terminal I is constant since, if there is any tendency for the output current to increase, the base current of transistor Q will tend to increase thus increasing the voltage drop across resistor 5 which elfectively lowers the base voltage on transistor Q which in turn decreases the base current of transistor Q therefore bringing the output current back to its constant value.

When the above circuit is fabricated as a monolithic integrated circuit all of the semi-conductors will have near identical temperature coeflicients. Therefore, they will track with each other almost exactly and will tend to compensate for temperature variations.

In order to calibrate the output current flowing at output terminal 1 the only adjustment needed is to vary the value of resistor 5.

The above circuit has been described as being particularly efficacious when manufactured as a monolithic integrated circuit since it can be easily and inexpensively fabricated as such. However, it is at once apparent that the circuit could be fabricated using discrete components in order to provide a high accuracy constant current source.

The invention has been described in detail with particular reference to a preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

I claim:

1. A constant current source adapted to be energized by a voltage source, said current source comprising:

(a) an input terminal adapted to be connected to the voltage source;

(b) first, second, and third unidirectional current control devices each having a collector, emitter, and a control electrode;

(c) a fourth unidirectional current control device;

(d) a common terminal;

(e) said input terminal being coupled to the control electrode of said first device and to said common terminal through said fourth device;

(f) a resistance;

(g) the collector of said first device being coupled to the control electrode of said second device, the emitter of said first device being coupled to the emitter of said third device and to said common terminal through said resistance:

(h) an output terminal adapted to be connected to a load, said output terminal being coupled to the emitter of said second device;

(i) the collector of said second device being coupled to the control electrode of said third device, the collector of said third device being coupled to said voltage source, so that the current through said resistor is equal to the sum of the currents through Refer Cit d said first and third devices. UNITED STATES PATENTS. 2. A constant current source as in claim 1 wherein said first and third devices are NPN transistors, and said 3,255,402 6/1966 Vounhalssecond device is a PNP transistor, and wherein each of 3310731 3/1967 Ostrofi et a1 said control electrodes is a base. 0 3,434,038 3/1969 Vette X 3. A constant current source as in claim 1 which is v fabricated as an integrated circuit. J D MILLER Pnmary Exammer 4. A constant current source as claimed in claim 1 GOLDBERG, Assistant EXaminel wherein said fourth device is an NPN transistor having [0 base, emitter, and collector electrodes, said base electrode U13 being connected to said collector electrode. 307297; 323-22, 38 

